Usually, both hydraulic and pneumatic actuator cylinders are designed to be used in such a way that the piston of the actuator cylinder moves from one end position to the other. In other words, the most important aspect is the effective movement created by the actuator cylinder, its positioning being of secondary importance. Thus, the actuator cylinder is operated using a simple control valve, which is a mechanical or electrical direction valve. However, the operating lever of the control valve must be kept turned the whole time until the piston reaches the desired position. In addition, actuator cylinders used for steering are quite small, so that even a small movement of the operating lever will move the piston rapidly for a long distance. This makes the steering jerky while the sudden movements can damage the structures attached to the actuator cylinder. Further, the control valve must be installed next to the operator, due to the small movement of the operating lever. This requires hose runs between the actuator cylinder and the control valve. In addition, the position of the piston cannot be predefined when using conventional control valves.
Particularly in steering applications, devices based on air springs or electric motors have also been developed, in which the operating lever can be turned directly to the desired position. These devices are, however, complicated and their use in, for instance, means of water transport demands especially watertight protection. In addition, the known devices are slow and demand a great deal of energy.
U.S. Pat. No. 3,915,070 discloses an actuator cylinder, in which there is also a detector element indicating the position of the piston. In the disclosed actuator cylinder, the longitudinal movement of the piston is converted to a rotating movement of the detector element, which shows the position of the piston in relation to the cylinder barrel. In this case too, the actuator cylinder is controlled using a conventional control valve, so that, despite the detector element, when used in steering the actuator cylinder moves in jerks. In addition, it is difficult to exploit the position data in steering, as the indicator connected to the detector element is located at the end of the actuator cylinder, or at least close to it.
U.S. Pat. No. 4,475,440 discloses one kind of actuator cylinder, in connection with which a control valve is arranged. This is a steering booster using particularly in vehicles. In the control valve, there is a special slide construction, to which the detector element is connected with the aid of a herringbone gear. Thus, when the piston reaches the position determined by the steering wheel, the slide construction has correspondingly rotated to the neutral position, guided by the detector element.
The actuator cylinder referred to above is large and contains a great many complicated components. In addition, the control valve is an extension of the actual cylinder barrel and contains a large number of pieces that are difficult to machine. In addition, the body of the control valve has several channels that must be machined, which complicates the manufacture of the actuator cylinder. Further, the detector element has several components and it has to be steadily fitted with a bearing to the cylinder barrel. Due to the numerous channels and complicated components, the flow resistance of the pressure medium is high. In addition, in the neutral position of the slide construction, the pressure is released on both sides of the piston. In other words, the actuator cylinder lacks a so-called holding property, so that external forces can affect the position of the piston.